The invention relates to a picture display device having a display tube with a rear part which accommodates a device for generating at least one electron beam and a front part which comprises a picture display phosphor screen, said display device also being provided with an electromagnetic deflection unit mounted around the display tube for deflecting electron beams across the display screen and comprising a line deflection coil and a field deflection coil which, when energized, generate magnetic fields having at least a dipole component.
Recently, more stringent standards have been introduced for certain types of picture display devices, notably for monitors, with respect to the magnetic interference field which they may produce around them. Protective shields have sometimes been used in picture display devices such as, for example, a metal cone envelope for the combination of display tube and deflection unit, but such protective shields are intended to inhibit the influence of external fields on the display device rather than reducing magnetic interference fields generated by the picture display device. An important source of magnetic interference fields is the line deflection coil, because it is operated at radio frequency currents (frequencies in the range of 10 to 100 kHz) as contrasted to the field deflection coil. It is impossible to design a satisfactorily operating deflection coil that produces no stray field. If the stray field were to be eliminated by means of a protective shield, such a shield would only be effective if the combination of display tube and deflection unit were also shielded on the display screen side.